1. Field of the Invention
The invention relates to determining distances. In particular, the invention relates to using one photon of a pair of entangled photons, reflected off an object, to determine a distance to that object.
2. Discussion of Background Information
Mankind has been interested in determining distances to remote objects for centuries. Presently known techniques for determining ranges include RADAR and LIDAR, which rely on detecting emitted electromagnetic radiation.
Photons are quanta of electromagnetic energy. Multiple photons may be entangled or not entangled. Photons that are not entangled together (i.e., random photons) exist as independent entities. In contrast, entangled photons have a connection between their respective properties.
Two photons entangled together are referred to as an entangled-photon pair (also, “biphotons”). Traditionally, photons comprising an entangled-photon pair are called “signal” and “idler” photons. Measuring properties of one photon of an entangled-photon pair determines results of measurements of corresponding properties of the other photon, even if the two entangled photons are separated by a distance. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of an entangled-photon pair cannot be factored into a product of two individual quantum states.
In general, more than two photons may be entangled together. More than two photons entangled together are referred to as “multiply-entangled” photons. Measuring properties of one or more photons in a set of multiply-entangled photons restricts properties of the rest of the photons in the set. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of a set of n>2 multiply-entangled photons cannot be factored into a product of n separate states. The term “entangled photons” refers to both biphotons and multiply-entangled photons.
Photon properties that may be entangled include time, frequency, polarization, and angular momentum. In particular, photons that are entangled in time are referred to as “temporally-entangled photons.” Such photons are generated nearly simultaneously. For given optical path lengths traveled by constituent photons in a temporally-entangled photon pair, detecting one of the photons places limits on the times at which the other photon may be detected. If the two temporally-entangled photons travel equal optical distances and the first photon is detected at time t0, then the second may only be detected within Te of t0, where Te is an entangled-photon parameter known as “entanglement time.”